1. Field of the Invention
The present invention relates to a circuit and a method for image frequency rejection.
2. Description of the Background Art
At present, there is a trend to carry out signal processing in the baseband by means of a digital signal processor. In narrow-band systems, several channels can be digitized by an analog-to-digital converter and the desired channel can be selected in the digital signal processor. In broad-band systems, the analog part of the receiver must be able to process the dynamic range of the analog signal. To this end, analog circuits are needed for image frequency signal rejection and frequency conversion.
Receiving systems with circuits according to FIG. 1a, FIG. 1b, and FIG. 1c are known from “A 2.4-GHz Low-IF Receiver for Wideband WLAN in 0.6 um CMOS-Architecture and Front-End,” Abidi et al., IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 35, No. 12, December 2000 and from “CMOS Mixers and Polyphase Filters for Large Image Rejection,” Abidi et al., IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 36, No. 6, June 2001. A front-end circuit for downmixing for a 2.4 GHz signal integrated into a receiver with a low intermediate frequency is also known. For dual frequency conversion, the analog receiver rejects the image frequency signal by 60 dB. FIG. 1a shows a fully integrated dual frequency-converting superheterodyne receiver with on-chip circuits for image frequency signal rejection. The circuit shows a dual conversion receiver with a low intermediate frequency (low IF). Input circuit 700 has an amplifier 702 (LNA, Low Noise Amplifier) with bandpass filter 701 and a quadrature mixer 710, whereby the quadrature mixer 710 for downmixing is connected to a polyphase filter 810 for generating an in-phase signal and a quadrature-phase signal from the first oscillator signal LO1. A dual polyphase filter 730 partially rejects the image frequency signal. Two amplifiers 711, 712 of an amplification device 710 are connected upstream of the dual polyphase filter 730.
The intermediate frequency mixer 300 converts the first intermediate frequency at its input into a low second intermediate frequency. In each case, five polyphase filters 111, 611 to 614 are connected downstream of four of the eight outputs of intermediate frequency mixer 300 in each case and polyphase filters 112, 621 to 624 of filter units 100, 600 are connected downstream of the other four outputs. Amplifiers 901, 902, 903, 904 of amplifier device 900 between polyphase filters 100 and 600 compensate for losses of about 10 dB in polyphase filters 100, 600. A programmable amplifier and an analog-to-digital converter can be connected downstream of polyphase filters 600 (not shown in FIG. 1).
A single-stage polyphase filter 611 used in the superheterodyne receiver is shown in FIG. 1b. Polyphase filter 611 has four resistors and four capacitors for phase shifting. A possible dimensioning of the resistors and capacitors is also given in the aforementioned publication.
Analog mixer 300, which downmixes the first intermediate frequency signal to the second intermediate frequency signal with a lower intermediate frequency, is formed as a dual-quadrature mixer 300. Dual-quadrature mixer 300 has four inputs for the differential in-phase component and the differential quadrature-phase component of the first intermediate frequency signal, which are multiplied by means of the dual-quadrature mixer 300 with an in-phase component ILO and a quadrature-phase component QLO of the oscillator signal LO2.
Because of the high requirements for pairing of the individual mixers for the image frequency signal rejection of 60 dB, the analog dual-quadrature mixer 300 is realized not by a Gilbert cell but by passive mixer 301 with four field-effect transistors in each case. A passive mixer 301 is shown schematically in FIG. 1c. Well-paired resistors of a resistor network 740 are used to adjust the amplification of passive mixers 301 of the dual-quadrature mixer 300 to one another.